'26 



THE NERVOUS SYSTEM 



the decussation of the jiyramids (tig. 43S) large Imndles of tilires pass from the 

 lateral columns of the cord to cross over to the o])posite side and ai)i)ear in the 

 anterior colunni of the medulla, forming the greater part of the pyramids. These 

 liundles of ti))res cut through the necks of the anterior horns so as to completely 

 amputate the heads of the horns. A little higher up each head is thrust into a 

 more lateral position by the increased size of the pyramids, and forms the nucleus 

 lateralis and the nucleus ambiguus (fig. 432). The motor nuclei of the portion of 

 the eleventh which arises in the medulla, and of the tenth and ninth nerves, are 

 regarded as derivatives from the head of the anterior horn, and the series is con- 

 tinued upwards by the nucleus of the seventh (at the junction of the pons and 

 medulla), and in the pons by the motor nucleus of the trigeniinal. A little above 

 the decussation of the pyramids, bundles of fibres, chiefly from the funiculi 

 gracilis and cuneatus, pass forwards and cut through the neck of the posterior horn, 

 and then decussate, forming the sensory or upper pyramidal decussation (decus- 

 satio lemnisci). The separated head of the posterior horn (substantia gelatinosa 

 Rolandi) is continued upwards as far as the accessory sensory nucleus of the fifth 

 nerve. This sul)stantia gelatinosa is one of the sensory nuclei of the fifth nerve, 

 and is covered externally by a l)and of white fibres, concave inwards, the ascend- 

 ing root of the trigeminal nerve (fig. 432). At one point the substantia gelatinosa 

 approaches very near the surface, and forms the grey tubercle of Rolando. 



Fig. 432. 



-Transverse .Sectiox of the Medulla ix thk Region' of the Decissatio 

 Lemxisci. (Sfhwalbe.) 



FcyiorLi's crxEATrs 



XrCLEl'S OF THE CLA VA 

 EXTERXAL CCXEATE XCCLEfS 



CUXEATE XUCLEiS 

 ASCEXDIXG ROOT OF 



TRIGEMIXAL / ( f' 



'AXTIA GELATIXOSA ■ I ' ':' " 



SrSSTA 



FORMATIO RETICULARIS 

 XrCLEUS LATERALIS 



OLIVARY XL' CLE US 



SUPERFICIAL ARCIFORM 

 FIBRES 

 ARCIFORM XUCLEUS 



FUXICULUS GRACILIS 

 POSTERIOR FISSURE 



XUCLEUS OF SPIXAL 



A CCESSOR Y 



— CEXTRAL CAXAL 



— XUCLEUS OF HYPOGLOSSAL 



— DECUSSA TIO LEMXISCI 



ACCESSORY OLIVARY 

 XUCLEUS 



I'-f AXTERIOR FISSU 



RE 



SUPERFICIAL ARCIFORM 

 FIBRES 



Of the nuclei which are apparently without representatives in the grey matter 

 of the spinal cord, the most important is the olivary nucleus ; this is a crumpled 

 bag of grey matter enclosing white matter and occupying the interior of the olivary 

 l)ody. It can easily be seen with the unaided eye in cross-sections of the medulla. 

 It presents a marked resemblance, in miniature, to the corpus dentatum of the 

 cerebellum, and is often called the corpus dentatum of the olivary body. It 

 |)resents an opening or hilum which opens downwards and inwards. The nucleus 

 of the clava and the nucleus cuneatus are not directly represented in the cord, 

 but their connection with the posterior horns can be seen in sections through the 

 lower part of the medulla; the}' have prol»al)ly the same anatomical value as a 

 eolunin of nerve-cells (Clarke's column) in the dorsal region of the s])inal cord 

 ( Testut). Other nuclei of smaller size are found in the medulla; the nucleus arci- 

 formis and the accessory olives will serve as examples of these (fig. 432). None of 

 these smaller nuclei are visil)le to the unaided eve. 



Specific Gravity and Weight of the Brain 



The white matter is soiut-wliat more 



The average specific gravity of the brain is about 1036 

 heavy (sp. gr. 1040) than the grey matter (sp. gr. 1034). 



